English

Every action of a non-amenable group is the factor of a small action

Dynamical Systems 2014-07-07 v2 Group Theory Logic

Abstract

It is well known that if GG is a countable amenable group and G(Y,ν)G \curvearrowright (Y, \nu) factors onto G(X,μ)G \curvearrowright (X, \mu), then the entropy of the first action must be greater than or equal to the entropy of the second action. In particular, if G(X,μ)G \curvearrowright (X, \mu) has infinite entropy, then the action G(Y,ν)G \curvearrowright (Y, \nu) does not admit any finite generating partition. On the other hand, we prove that if GG is a countable non-amenable group then there exists a finite integer nn with the following property: for every probability-measure-preserving action G(X,μ)G \curvearrowright (X, \mu) there is a GG-invariant probability measure ν\nu on nGn^G such that G(nG,ν)G \curvearrowright (n^G, \nu) factors onto G(X,μ)G \curvearrowright (X, \mu). For many non-amenable groups, nn can be chosen to be 44 or smaller. We also obtain a similar result with respect to continuous actions on compact spaces and continuous factor maps.

Keywords

Cite

@article{arxiv.1311.0738,
  title  = {Every action of a non-amenable group is the factor of a small action},
  author = {Brandon Seward},
  journal= {arXiv preprint arXiv:1311.0738},
  year   = {2014}
}